Polyvinyl acetate latex

ABSTRACT

The present invention relates to a process for preparing an aqueous dispersion of a vinyl acetate polymer comprising the step of polymerizing vinyl acetate under emulsion polymerization conditions in the presence of a chain transfer agent which is a hypophosphite salt or X—R 1 —SH, where R 1  is a C 1 -C 4  alkyl group and X is sulfonate, hydroxyl, sulfate, phosphate, phosphonate, carboxylic acid or a salt thereof, or C 1 -C 3 -alkyl carboxylate. The process provides a way of lowering the viscosity of the vinyl acetate polymer at a given solids content.

BACKGROUND OF THE INVENTION

The present invention relates to a polyvinyl acetate latex composition, more particularly, a relatively low viscosity polyvinyl acetate latex composition.

Historically, polyvinyl acetate containing latexes develop high in-process viscosities, which present great challenges to processing and manufacturing. Failure to reduce in-process viscosity often forces latex manufacturers to either use extreme agitation, which may present safety issues in a plant, or to reduce polymer solids, which adds unnecessary cost for shipping and handling. Furthermore, low solid content latexes limit the marketability of the latex. Accordingly, it would advantageous to develop polyvinyl acetate latexes with higher solids and/or lower viscosities.

SUMMARY OF THE INVENTION

The present invention is a process for preparing an aqueous dispersion of a vinyl acetate polymer comprising the step of polymerizing vinyl acetate under emulsion polymerization conditions in the presence of a chain transfer agent which is a hypophosphite salt or X—R¹—SH, where R¹ is a C₁-C₄ alkyl group and X is sulfonate, hydroxyl, sulfate, phosphate, phosphonate, carboxylic acid or a salt thereof, or C₁-C₃-alkyl carboxylate. The process of the present invention provides a way to prepare vinyl acetate homopolymers and copolymers with lower end-of-feed viscosities at a given solids content.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is a process for preparing an aqueous dispersion of a vinyl acetate polymer comprising the step of polymerizing vinyl acetate under emulsion polymerization conditions in the presence of a chain transfer agent which is a hypophosphite salt or X—R¹—SH, where R¹ is a C₁-C₄ alkyl group and X is sulfonate, hydroxyl, sulfate, phosphate, phosphonate, carboxylic acid or a salt thereof, or C₁-C₃-alkyl carboxylate. The vinyl acetate polymer may be a homopolymer or a copolymer; therefore the polymer may be prepared by homopolymerization or copolymerization in the presence of another monomer, such as acrylates and methacrylates including methyl methacrylate, butyl acrylate, ethyl acrylate, ethylhexyl acrylate, and ureido methacrylate; vinyl esters of a branched carboxylic acid monomer, including vinyl versatate; phosphorus acid monomers such as phosphoethyl methacrylate, phosphopropyl methacrylate, and CH₂═CH—CH₂—(OCH₂CH₂)_(n)(O)_(m)—P(O)(OH)₂, or a salt thereof, where n is from 1 to 5 and m is 0 or 1; and acrylamide monomers and sulfonic acid monomers and salts thereof and combinations thereof including acrylamide, 2-acrylamido-2-methylpropane sulfonic acid or a salt thereof, and vinyl sulfonic acid or a salt thereof.

Preferably, the concentration of vinyl acetate is from 40, more preferably from 50, and most preferably from 60 weight percent, to preferably 95, and more preferably to 90 weight percent based on the weight of total monomers.

A low T_(g) alkyl acrylate such as butyl acrylate, ethyl acrylate, or ethylhexyl acrylate is preferably included as a comonomer in the polymerization step at a concentration in the range of from 5, more preferably from 8, and most preferably to 10 weight percent, to 50, more preferably to 40, and most preferably to 35 weight percent, based on the weight of total monomers.

When a phosphorus acid monomer is used, it is preferably included at a concentration in the range of from 0.1, more preferably from 0.2 weight percent, to preferably 5, more preferably to 3, and most preferably to 2 weight percent based on the weight of total monomers. A preferred phosphorus acid monomer is phosphoethyl methacrylate (PEM), which is characterized by the following formula:

where R is H or

wherein the dotted line represents the point of attachment to the oxygen atom.

2-Acrylamido-2-methylpropane sulfonic acid or a salt thereof or vinyl sulfonic acid or a salt thereof, preferably 2-acrylamido-2-methylpropane sulfonic acid or a salt thereof, is preferably included as a comonomer at a concentration in the range of from 0.1, more preferably from 0.5 weight percent, to 5, more preferably to 3, and most preferably to 2 weight percent, based on the weight of total monomers.

Ureido methacrylate may also be included at a concentration in the range of from 0.1, more preferably from 0.2, and most preferably from 0.5 weight percent, to 5, more preferably to 3, and most preferably to 2 weight percent, based on the weight of total monomers.

Examples of suitable chain transfer agents include sodium and potassium hypophosphite salts, sodium and potassium salts of β-mercaptoethanesulfonate; γ-mercaptopropionic acid and sodium and potassium salts thereof; and β-mercaptoethanol. The chain transfer agent is preferably used at a concentration of from 0.01, more preferably from 0.02, and most preferably from 0.05 weight percent, to preferably 2, more preferably to 1, and most preferably to 0.5 weight percent, based on the total weight of monomers.

It has been discovered that the aqueous dispersion of the vinyl acetate polymer (the PVA latex) prepared by the process of the present invention, preferably the vinyl acetate copolymer, exhibits a lower viscosity in the aqueous phase than comparable vinyl acetate polymers prepared by processes that do not include the specified class of chain transfer reagents. It is believed that hypophosphites and water-soluble alkyl mercaptans reduce the weight average molecular weight /(M_(w)) of water-soluble polymers, which accounts for a reduced in-process latex viscosity at constant solids content (or an increased solids content at comparable viscosity) measured against PVA latexes that do not use chain transfer agents within the specified class. Lowering the viscosity in a latex formulation without lowering solids content provides for a more efficient use of water in a latex formulation.

The PVA latex is useful as a binder for coatings compositions, adhesives, or construction materials. For coating compositions, the latex can combined with one or more ingredients selected from the group consisting of pigments such as TiO₂, dispersants, defoamers, surfactants, solvents, additional binders, thickeners, extenders, coalescents, biocides, and colorants.

Abbreviations

Abbreviation Name BA Butyl Acrylate VA Vinyl Acetate AMPS 2-Acrylamido-2-methylpropane sulfonic acid PEM Phosphoethyl Methacrylate t-BHP t-Butyl Hydroperoxide IAA Isoascorbic Acid UMA Ureido Methacrylate EA Ethyl Acrylate MMA Methyl Methacrylate MAA Methacrylic Acid NaPS Sodium Persulfate PS Particle Size

EXAMPLES

In the following examples, the average particle size was measured using dynamic light scattering at 90° on a Brookhaven 90Plus Particle Size Analyzer. The viscosity was measured on either a Brookfield DV-II Pro Viscometer or a Brookfield LVTD Viscometer.

Comparative Example 1 Preparation of PVA Copolymer without any Chain Transfer Agent

The monomer emulsion was prepared by first mixing DI water (2258 g), PEM (60% active, 230.85 g), NaOH (50%, 151 g), Disponil FES-77 surfactant (185.37 g), TERGITOL™ 15-S-40 surfactant (169.2 g), and dodecylbenzene sulfonate (22.5% aq., 220.35 g) followed by BA (1370.63 g), VA (9810.90 g) and Lubrizol AMPS 2405 (259.7 g).

A 5-gal (19-L) reactor equipped with a mechanical stirrer, nitrogen gas blanket, thermometer, condenser, heating mantel, and temperature controller was charged with DI water (9386.8 g) and heated to 71° C. under N₂. To this flask were added Disponil FES-32 surfactant (250.08 g), TERGITOL™ 15-S-40 surfactant (169.2 g), and a solution of FeSO₄ heptahydrate (0.36 g) in water (20 g). Co-feed catalyst solution t-BHP (37.1 g) in water (222.6 g) and co-feed activator solution IAA (51.08 g) and sodium acetate (35.67 g) in water (482.18 g) were fed to the flask at a rate of 1.44 g/min, and 2.96 g/min, respectively. Two minutes later, monomer emulsion was fed to the flask at a rate of 64.78 g/min. Ten minutes later, the monomer emulsion feed rate was increased to 129.55 g/min and the reaction temperature was controlled at 71° C. After the monomer emulsion addition was complete, the monomer emulsion vessel was rinsed with DI water (160 g), and the co-feed solutions were continued for an additional 60 min until completion. After the co-feed solutions were complete, a solution of t-BHP (12.36 g) in water (74.21 g) and a solution of sodium bisulfite (15.38 g) in water (161.56 g) were fed separately to the flask at 1.44 g/min and 2.95 g/min, respectively, to reduce the residual monomer. After completion of all feeds, the flask was cooled to room temperature. When the flask was cooled to 50° C., NH₃ (52.46 g, 28% aq.) was added to the flask at 10.49 g/min over 5 min, then cooled to 40° C., whereupon a solution of KATHON™ LX bactericide (24.74 g, 1.5%) in water (60 g) was added at a rate of16.95 g/min over 5 min. After cooled to room temperature, the contents were filtered to remove gel. The filtered dispersion was found to have a solids content of 44.0% and a pH of 6.

Example 1 Preparation of PVA Copolymer Sodium hypophosphite Chain Transfer Agent

The monomer emulsion was prepared by first mixing DI water (2258 g), a sodium hypophosphite solution (26.5 g, 45% aq), phosphoethyl methacrylate (60% active, 230.85 g), NaOH (50%, 151 g), Disponil FES-77 surfactant (185.37 g), TERGITOL™ 15-S-40 surfactant (169.2 g), and dodecylbenzene sulfonate (22.5% aq., 220.35 g) followed by BA (1255.21 g), VA (9810.90 g) and Lubrizol AMPS 2405 (259.7 g).

A 5-gal (19-L) reactor equipped with a mechanical stirrer, nitrogen gas blanket, thermometer, condenser, heating mantel and temperature controller was charged with deionized water (9160 g) and heated to 71° C. while purged with N₂. To this flask were added Disponil FES-32 surfactant (250.08 g), TERGITOL™ 15-S-40 surfactant (169.2 g), and a solution of FeSO₄ heptahydrate (0.36 g) in water (20 g). Co-feed catalyst solution t-BHP (37.1 g) in water (449 g) and a co-feed solution of IAA (51.08 g) and sodium acetate (35.67 g) in water (482.18 g) were fed to the flask at a rate of 2.7 g/min, and 2.96 g/min, respectively. Two minutes later, monomer emulsion was fed to the flask at a rate of 64.78 g/min. Ten minutes later, the monomer emulsion feed rate was increased to 129.55 g/min and the reaction temperature was controlled at 71° C. 60 min after the start of the monomer emulsion feed, UMA (50% active, 230.84 g) was added to the monomer emulsion. After the monomer emulsion addition was complete, the monomer emulsion vessel was rinsed with DI water (160 g), and the co-feed solutions were continued for an additional 60 min until completion. After the co-feed solutions were complete, a solution of t-BHP (12.36 g) in water (74.21 g) and a solution of sodium bisulfite (15.38 g) in water (161.56 g) were fed separately to the flask at 1.44 g/min and 2.95 g/min, respectively, to reduce the residual monomer. After completion of all feeds, the flask was cooled to room temperature. When the flask was cooled to 50° C., NH₃ (52.46 g, 28% aq.) was added to the flask at 10.49 g/min over 5 min. The reactor was cooled to 40° C., whereupon a solution of KATHON™ LX bactericide (24.74 g, 1.5%) in water (60 g) was added at a rate of 16.95 g/min over 5 min. After the reactor was cooled to room temperature, the contents were filtered to remove gel. The filtered dispersion was found to have a solids content of 44.3% and a pH of 6.

Example 2 Preparation of PVA Copolymer with Sodium hypophosphite Chain Transfer Agent

The monomer emulsion was prepared by first mixing DI water (2258 g), a sodium hypophosphite solution (26.5 g, 45% aq), PEM (60% active, 230.85 g), NaOH (50%, 151 g), Disponil FES-77 surfactant (185.37 g), TERGITOL™ 15-S-40 surfactant (169.2 g), and dodecylbenzene sulfonate (22.5% aq., 220.35 g) followed by BA (1255.21 g), VA (9810.90 g) and Lubrizol AMPS 2405 (259.7 g).

A 5-gal (19-L) reactor equipped with a mechanical stirrer, nitrogen gas blanket, thermometer, condenser, heating mantel and temperature controller was charged with deionized water (8100 g) and heated to 71° C. while purged with N₂. To this flask were added Disponil FES-32 surfactant (250.08 g), TERGITOL™ 15-S-40 surfactant (169.2 g), and a solution of FeSO₄ heptahydrate (0.36 g) in water (20 g). Co-feed catalyst solution t-BHP (37.1 g) in water (449 g) and co-feed solution of IAA (51.08 g) and sodium acetate (35.67 g) in water (482.18 g) were fed to the flask at a rate of 2.7 g/min, and 2.96 g/min, respectively. Two minutes later, the monomer emulsion was fed to the flask at a rate of 64.78 g/min. Ten minutes later, the monomer emulsion feed rate was increased to 129.55 g/min and the reaction temperature was controlled at 71° C. After the monomer emulsion addition was complete, the monomer emulsion vessel was rinsed with DI water (160 g), and the co-feed solutions were continued for an additional 60 min until completion. After the co-feed solutions were complete, a solution of t-BHP (12.36 g) in water (74.21 g) and a solution of sodium bisulfite (15.38 g) in water (161.56 g) were fed separately to the flask at 1.44 g/min and 2.95 g/min, respectively, to reduce the residual monomer. After completion of all feeds, the flask was cooled to room temperature. When the flask was cooled to 50° C., NH₃ (52.46 g, 28% aq.) was added to the flask at 10.49 g/min over 5 min. The reactor was cooled to 40° C., whereupon a solution of KATHON™ LX bactericide (24.74 g, 1.5%) in water (60 g) was added at a rate of 16.95 g/min over 5 min. After the reactor was cooled to room temperature, the contents were filtered to remove gel. The filtered dispersion was found to have a solids content of 46.1% and a pH of 6.

Table 1 illustrates the relationship between solids content and the end of the monomer emulsion feed viscosity (EOF) at the polymerization reaction temperature. Cl refers to Comparative Example 1, and 1 and 2 refer to Examples 1 and 2, respectively. PS refers to particle size of the latex particles; EOF refers to end of feed viscosity in centipoise (cP) and was measured using a Brookfield DV-II Pro Viscometer.

TABLE 1 Effect of Sodium Hypophosphite CTA on PVA Latex Copolymer Viscosity Ex. Solids PS NaH₂PO₃ EOF # (%) (nm) (%) (cP) Composition C1 44 117 0 2500 85 VA/11.88 BA/2 PEM/ 1.12 AMPS 1 44.3 122 0.1% 768 85 VA/10.88 BA/2 PEM/ 1 UMA/1.12 AMPS 2 46.1 124 0.1% 2240 86 VA/10.88 BA/2 PEM/ 1.12 AMPS

Table 1 shows that a small addition of sodium hypophosphite causes a marked reduction in EOF at the same solids content and approximately the same particle size, and even a 10% reduction in EOF with a significantly improved solids content (46.1 versus 44% solids).

The PVA latex containing structural units of VA, BA, and AMPS—but without PEM—was also evaluated with and without sodium hypophosphite (NaH₂PO₃) as shown in Table 2. Comparative Example 2 can be prepared by conventional methods such as described for Comparative Example 1.

TABLE 2 Effect of Sodium Hypophosphite CTA on PVA Latex Copolymer Viscosity Ex. Solids PS NaH₂PO₃ EOF # (%) (nm) (%) (cP) Composition C2 56.2 223 0 22800 65.4 VA/34 BA/0.6 AMPS 3 56.4 218 0.1 9400 65.4 VA/34 BA/0.6 AMPS

Table 2 illustrates that the viscosity drop at constant solids content is observed for PVA latex copolymers that do not contain phosphorus acid functionalization.

Comparative Example 3 Preparation of a VA Copolymer without a Chain Transfer Agent

Monomer emulsion was prepared by first mixing DI water (368.8 g), PEM (60% active, 8.53 g), NaOH (50%, 5.29 g), TERGITOL™ 15-S-9 surfactant (A Trademark of The Dow Chemical Company or Its Affiliates, 17.1 g), Disponil FES-77 surfactant (27.38 g),), TERGITOL™ 15-S-40 surfactant (49.93 g), and dodecylbenzene sulfonate (22.5% aq., 32.55 g) followed by BA (570.15 g), VA (1116.78 g) and Na-AMPS (19.1 g).

A 5-L 4-necked round bottom flask equipped with a mechanical stirrer, nitrogen gas blanket, thermometer, condenser, heating mantel and temperature controller was charged with deionized water (900 g) and heated to 71° C. while purged with N₂. To this flask was added a solution of FeSO₄ heptahydrate (0.1% in water, 18g), nitrilotriacetic acid (0.1% in water, 15 g) and EA/MMA/MAA pre-form seed (52/47/1, 40 nm particle size, 103.83 g). Co-feed catalyst (t-BHP, 1.51 g) and NaPS (2.27 g) in water (49.5 g) and co-feed activator (IAA, 2.26 g) and NaOH (1.02 g, 50% aq.) in water (50 g) were fed to the flask at a rate of 0.34 g/min. Five minutes later, the monomer emulsion was fed to the flask at a rate of 9.6 g/min. Fifteen minutes later, the monomer emulsion feed rate was increased to 19.2 g/min and the reaction temperature was controlled at 71° C. After the monomer emulsion addition was complete, the monomer emulsion vessel was rinsed with DI water (40 g), and the co-feed solutions feed rates were decreased to 0.25 g/min. Feed was continued for an additional 45 min until completion. A solution of t-BHP (0.48 g) and H₂O₂ (8.25 g) in water (42 g) and a solution of IAA (5.12 g) and NaOH (2.3 g, 50% aq.) in water (42 g) were fed separately to the flask at 0.85 g/min to reduce the residual monomer. After completion of addition of all feeds, the flask was cooled to room temperature. When the flask temperature reached 50° C., a solution of NH₃ (2.9 g, 28% aq.) in water (8 g) was added to the flask dropwise. The flask was cooled to 45° C., whereupon a solution of ROCIMA™ BT 2S bactericide (A Trademark of The Dow Chemical Company or Its Affiliates, 1.87 g, 19.3% aq.) in water (10 g) and a solution of KORDEK™ LX5000 bactericide (A Trademark of The Dow Chemical Company or Its Affiliates, 50%, in 10 g water) was added dropwise. After the flask was cooled to room temperature, the contents were filtered to remove gel. The filtered dispersion was found to have a solids content of 51.42% and a pH of 6.

Examples 4-8 and Comparative Examples 4-6 were carried out substantially as described for Comparative Example 3 except for the inclusion of chain transfer agents of the monomer emulsion in the amounts described in Table 3. Comparative Examples 4-6 show examples of chain transfer agents not within the scope of the present invention.

TABLE 3 The Effect of Various Types of CTAs on PVA Copolymer Latex Viscosity Ex. Solids PS Concentration/ EOF # (%) (nm) CTA (cP) composition 4 51.8 163.4 mercapto- 1200 65.5 VA/33.44 BA/ ethanesulfonate 0.56 AMPS/0.5 PEM 0.13%, 13 mmol 5 51.4 163.9 mercaptopropionic 2480 65.5 VA/33.44 BA/ acid 0.56 AMPS/0.5 PEM 0.1%, 17 mmol 6 51.3 164.5 Sodium 3020 65.5VA/33.44 BA/ Hypophosphite 0.56 AMPS/0.5 PEM 0.08%, 13 mmol 7 50.8 169.1 mercaptoethanol 2440 65.5 VA/33.44 BA/ 0.07%, 13 mmol 0.56 AMPS/0.5 PEM 8 51.4 167.2 Methyl 3000 65.5 VA/33.44 BA/ mercaptopropionate 0.56 AMPS/0.5 PEM 0.09%, 13 mmol C3 51.4 168.1 0 4200 65.5 VA/33.44 BA/ 0.56 AMPS/0.5 PEM C4 51.6 164 n-Dodecyl 3880 65.5 VA/33.44 BA/ mercaptan 0.56 AMPS/0.5 PEM 0.15% 12 mmol C5 51.0 166.9 Chloroacetic acid, 5100 65.5VA/33.44 BA/ sodium salt 0.56AMPS/0.5 PEM 0.1% 15.4 mmol C6 51.0 166.7 Bromoacetic acid 5920 65.5VA/33.44 BA/ 0.1%, 13 mmol 0.56 AMPS/0.5 PEM

The data show that PVA latex copolymers prepared using CTAs within the scope of the present invention exhibit end of feed viscosities that are lower and, in some cases, significantly lower, than comparable copolymers that contain either no CTA (Comparative Example 3) or n-dodecyl mercaptan (Comparative Example 4) or chloroacetic acid, sodium salt (Comparative Example 5) or bromoacetic acid. 

1. A process for preparing an aqueous dispersion of a vinyl acetate polymer comprising the step of polymerizing vinyl acetate under emulsion polymerization conditions in the presence of a chain transfer agent which is a hypophosphite salt or X—R¹—SH, where R¹ is a C₁-C₄ alkyl group and X is sulfonate, hydroxyl, sulfate, phosphate, phosphonate, carboxylic acid or a salt thereof, or C₁-C₃-alkyl carboxylate.
 2. The process of claim 1 wherein the vinyl acetate polymer is a copolymer which is prepared by the copolymerization of vinyl acetate and at least one additional monomer which is an acrylate, a methacrylate, a phosphorus acid monomer, an acrylamide, a sulfonate monomer, or a vinyl ester of a branched carboxylic acid monomer, or combinations thereof, wherein the concentration of the chain transfer agent is from 0.01 to 1 weight percent, based on the weight of total monomers.
 3. The process of claim 2 wherein the copolymer is prepared by the copolymerization of from 50 to 95 weight percent vinyl acetate and from 5 to 50 percent butyl acrylate based on the weight of total monomers.
 4. The process of claim 2 wherein the copolymer is prepared by the copolymerization of monomers comprising from 50 to 90 weight percent vinyl acetate, from 5 to 40 weight percent butyl acrylate, from 0.2 to 3 weight percent of phosphoethyl methacrylate, and from 0.1 to 3 weight percent 2-acrylamido-2-methylpropane sulfonic acid or a salt thereof, based on the weight of total monomers, wherein the chain transfer agent is sodium hypophosphite, mercaptoethanesulfonate, mercaptopropionic acid or a salt thereof, mercaptoethanol, or methyl mercaptoproprionate.
 5. The process of claim 4 wherein the chain transfer agent is sodium hypophosphite.
 6. The process of claim 4 wherein the chain transfer agent is mercaptoethanesulfonate, mercaptopropionic acid or a salt thereof, mercaptoethanol, or methyl mercaptopropionate.
 7. The process of claim 5 which further comprises from 0.2 to 2 weight percent ureido methacrylate based on the weight of total monomers.
 8. The process of claim 2 wherein the copolymer is prepared by the copolymerization of monomers comprising from 50 to 90 weight percent vinyl acetate, from 5 to 40 weight percent butyl acrylate, and from 0.1 to 2 weight percent 2-acrylamido-2-methylpropane sulfonic acid or a salt thereof or sodium vinyl sulfonate, based on the weight of total monomers, wherein the chain transfer agent is sodium hypophosphite.
 9. The process of claim 1 which further includes the step of combining the vinyl acetate polymer with one or more ingredients selected from the group consisting of pigments, dispersants, defoamers, surfactants, solvents, additional binders, thickeners, extenders, coalescents, biocides, and colorants. 